Pressing plate for linearized pulses from a peristaltic pump

ABSTRACT

A peristaltic pump comprises a rotatable drum 2 having rollers 5 or cams 5&#39; for squashing a flexible tube 9 against a profiled surface 10 of a presser plate 7. Tube 9 is mounted between supports 14 and 15 on presser plate 7 and is automatically movable between an inoperative and operative (pumping) position by pivotal movement of plate 7 about axis 8 produced by an electro mechanical actuator. A flexible membrane 33 is sandwiched between rollers 5 or cams 5&#39; for eliminating shear forces on the tube 9. The construction of the pump and shape of profile 10 are such as to mimimise pulsations in the output flow. Systems for supplying a sample for analysis to spectroscopic apparatus using the peristaltic pump are also described.

This application is a division of application Ser. No. 08/378,028, filedJan. 24, 1995.

TECHNICAL FIELD

This invention relates to peristaltic pumps and particularly but notexclusively such pumps intended for use in circumstances where theamount of material pumped over a given time period needs to beaccurately controlled. One such circumstance is the presentation tospectroscopic apparatus of a sample to be analysed by that apparatus.This invention also relates to spectroscopic analysis of substances andis particularly concerned with a system and method whereby samples arepresented for analysis by spectroscopic apparatus. It will be convenientto hereinafter describe the invention in relation to spectroscopicapparatus, but it is to be understood that a pump according to theinvention has other applications.

BACKGROUND

Peristaltic pumps are well known and comprise a rotatable drum having aplurality of rollers located around its periphery, and a flexible tubewhich is held against the drum periphery by a presser plate and throughwhich liquid (for example) is pumped. The tube is held by two spacedfixed supports, and liquid is caused to move through the tube inresponse to a moving pinch zone caused by the rollers pressing on thetube as the drum rotates.

Pumps of the foregoing kind suffer problems which tend to make themunsuitable for use in certain circumstances. One such problem is atendency for the tube to become distorted and to take on a permanent setwhich makes it unsuitable for further work. In operation, the tube ispulled over the drum and is attached to the fixed supports so as to havea correct amount of longitudinal tension. If the tube is left in theoperating position for extended periods of time with the pumpstationary, the aforementioned distortion may occur. Consequently, it isgood practice to release the presser plate and the tube tension at theend of each period of use, but that is often overlooked either byaccident or by design, particularly as the re-establishment of the pumpto an operative condition is a tedious process.

Another problem with peristaltic pumps is that the output flow is apulsating flow, and that tends to make such pumps unsuitable for use insome circumstances. For example, the pulsating flow makes such pumpsunsuitable for use in delivering a sample to be analysed to thenebulizer of a spectroscopic instrument. That difficulty can be met byoperating the pump at a very high speed but such operation is not alwayspossible or convenient.

Another problem in peristaltic pumps that are to be used for accuratelymetering materials and which is a significant component in their cost,is that dimensional tolerances in the manufacture of individualcomponents, such as the drum and the rollers, and positional toleranceson assembling the different parts are necessarily quite small to ensurethe requisite accuracy in operation of the pump. Also the rollerbearings in known pumps often corrode and produce unreliable operation.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a peristaltic pumpin which one or more of the aforementioned problems are ameliorated. Itis another object of an embodiment of the invention to provide aperistaltic pump of relatively simple construction and which isrelatively inexpensive to manufacture.

A peristaltic pump according to the broadest aspect of the invention ischaracterised in that at least one of the tube supports is movablerelative to the drum between loaded and unloaded positions rather thanhaving a fixed position relative to the drum as in prior constructions.

Thus, according to this broadest aspect of the invention, there isprovided a peristaltic pump including a rotatable drum includingcompressing elements around its periphery, a flexible tube extendingbetween two spaced supports and a movable presser plate for holding thetube against the periphery of the drum between the two spaced supportssuch that on rotation of the drum the compressing elements squeeze thetube against the presser plate for moving fluid through the tube,wherein at least one of the tube supports is movable relative to thedrum to establish an operative and an inoperative position for the tube,the operative position being when the tube is positioned for pumping.

Preferably, the movable support is fixed to or formed integral with thepresser plate so as to move with that plate between the loaded andunloaded positions.

It has been conventional practice in the past to secure the presserplate in the pump operative position by means of a spring influencedclamp or the like. According to a preferred form of the presentinvention, the presser plate is moved into the operative position bymeans of an electro-mechanical actuator. It is further preferred thatthe arrangement is such that the presser plate is subjected to a controlsystem such that it can be moved automatically between operative andinoperative positions thereof. Thus, in this preferred arrangement, aperistaltic pump is provided in which the tube tension is automaticallyestablished as the pump is being conditioned for operation and isautomatically de-established at the end of each operating sequence.

A further aspect of the invention is that the known rotatable drum androllers assembly in a peristaltic pump according to the invention may bereplaced by a cam, the profile of which comprises a plurality of cammingsurfaces for squeezing the tube. Thus a pump according to the inventionwill contain tube compressing elements which may be in the form ofrollers or fixed camming surfaces around the periphery of the drum.

A further feature in a pump according to the invention is that thepresser plate may be profiled in such a way as to minimise pulsationsand lack of stability in the output flow. That profiling is applied tothe surface of the presser plate which is opposed to the drum and istherefore the surface which holds the tube against the drum rollers orcamming surfaces. Preferably the profiled presser plate surface includesor is composed of at least two regions, a pinch region and an expansionregion which is located downstream of the pinch region in relation tothe direction of flow through the tube. The pinch region is arranged tocooperate with the drum rollers or camming surfaces so that theaforementioned pinch zones are created within that region. The expansionregion preferably follows immediately after the pinch region and isarranged so that the space between it and the drum progressivelyincreases in the downstream direction. In one arrangement, there is athird region upstream of the pinch region, which is called an entranceregion and which is arranged so that the space between it end the drumprogressively decreases in the downstream direction.

The operation of a peristaltic pump is essentially equivalent to movinga restriction along the length of the tube so that liquid within thetube and in advance of the restriction is pushed through the tube by therestriction. There is in fact a plurality of such restrictions which arecreated in sequence by the rollers or camming surfaces as they are movedagainst that section of the tube which is influenced by the presserplate. As each roller or cam moves away from that section of the tube,it allows the tube to expand and thereby increase the internal volume ofthe tube downstream of the presser plate. That expansion gives rise topulsations in the output flow of the pump.

Adoption of a presser plate having an expansion region as describedabove enables output flow pulsations to be eliminated, or at leastreduced. This is achieved by designing the profile of the expansionregion so that a substantially linear relationship exists between theangular rotation of the drum and the increase in the tube internalvolume resulting from withdrawal of a drum roller or cam from the tube.Ideally the linear relationship should be such as to increase theinternal volume by the volume contained by a single tube pinch over anangular rotation equivalent to the angular separation of one roller orcamming surface from the next, and then the output flow is free ofpulsations. In other circumstances, a strictly linear relationship maynot be possible, but it is nevertheless possible to reduce the amplitudeof the pulsations to an acceptable level by means of profiling asdiscussed above.

The presser plate may furthermore be profiled such that over a portionof said profile, the gap between the camming surfaces, or rollers of therotating drum, and said portion is less than that which is necessary toseal the tube. In operation, the camming surfaces or rollers"over-squash" the tube, thus allowing for a lesser degree of accuracy insizing and assembling the pump parts in manufacture, although theprimary aim of this feature is to allow a lesser degree of relativepositioning accuracy of the pump parts.

Another cause of pulsation in the output flow in pump constructionswherein the compressing elements are rollers is frictional resistance torotation of the rollers. Each roller rotates in response to a tangentialforce created between it and the pump tube, which is held againstmovement with the drum. That force tends to stretch the tubelongitudinally and also generates shear forces in the tube because theside of the tube contacting the presser plate does not experience thesame tangential forces. Such shear forces contribute to tube wear andfatigue failure. Also, as each roller withdraws from the tube, the tubeis able to relax longitudinally and thereby cause a change in theinternal volume of the tube such as to introduce pulsations into theoutput flow.

Prior attempts to meet the problem have not been entirely satisfactory.One approach has been to reduce frictional resistance to roller rotationby mounting the rollers on ball bearings, but that is a complex andcostly approach which alleviates the problem rather than solves it.Another approach has been to drive the rollers through a planetary gearsystem, and that is a very expensive approach which also fails to solvethe problem. In order to be effective, the rotational speed of therollers must exactly match the traverse speed of the drum over the tube,but such matching is seldom achieved.

According to a further preferred feature of the invention, theaforementioned problem is met or at least substantially alleviated byinterposing a flexible membrane between the drum and the tube such thatthe membrane rather than the tube absorbs the aforementioned shearforces. The membrane is anchored upstream of the region within which thetube is pinched as previously described, and is preferably composed of amaterial which is not prone to stretch in the longitudinal direction ofthe tube. Suitable materials include polyester such as Mylar and otherplastics materials, and metal foil, but that is not an exhaustive list.

In a pump having camming surfaces instead of rollers, the frictionalforces on the tube will be larger. Thus in this form of pumpconstruction a flexible membrane needs to be interposed between the camand the tube for absorbing shear forces that would otherwise act on thetube. It is desirable for the contacting surfaces of the cam and themembrane to be lubricated and such lubrication may be provided by alubricating substance, for example a silicone grease, placed on themembrane or camming surfaces. Alternatively the membrane or cammingsurfaces may be self lubricating. Preferably the membrane is of asubstance or constructed such that its cam facing surface is veryslippery, for example the membrane may be a laminate of Mylar and a moreslippery plastic.

When a flexible membrane is disposed between the drum and the tube asdescribed above, the tension applied to the tube when it is in itsoperative position is preferably very little (that is, it is close tozero or even zero) so that the elasticity of the tube defines a volumeof liquid between adjacent rollers or cams which is independent ofroller (or cam) speed. Thus, the tube is preferably held reasonablyloosely in its operative position and there is minimal or zerostretching of the tube.

Where the presser arm force is generated by an electromechanicalactuator such as a stalled DC motor run at a controlled current,friction or other effects in the actuator may cause uncertainty in theoutput torque and hence presser plate force. This uncertainty isundesirable and may be overcome by cycling the current setpoint aboutits mean value at a rate fast enough to not affect operation of thesystem, yet slow enough for the actuator to respond.

One technique employed in spectroscopy is to produce a sample solutioncontaining the substance of interest, and to introduce that solutioninto a nebulizer which directs an atomized body of the solution into aflame or plasma. Preparation of such samples is an exacting and timeconsuming operation, and obtaining a suitable level of dilution is oneparticular problem in that preparation. In addition to the samplesolutions, it is necessary to prepare standard solutions for comparativereference, and that adds further to the time required to conduct thesample analysis.

There have been several proposals for overcoming or alleviating all orsome of the foregoing problems. One such proposal is that of Jones asdescribed in "Atomic Absorption Newsletter", Vol. 9, No. 1,January-February 1970, pages 1 to 5. The Jones proposal involvesconnecting separate sample solution and diluent supplies to thenebulizer through a T junction, and delivering the sample to thatjunction by way of a variable speed syringe pump. Fluid flows into thenebulizer at the natural aspiration rate of the system. The flow rate ofthe sample is controlled by the pump speed, and the flow of diluentautomatically adjusts so that the total flow rate is constant. Suitablevariation of the pump speed then results in achievement of a desiredlevel of dilution in the sample stream presented to the nebulizer.

A deficiency of the Jones proposal is that it utilizes the same pump forstandard and sample solutions respectively. In particular, it isnecessary to replace the supply of one solution with the supply sourceof the other when it is required to switch from analysis of one solutionto the other. It also uses a syringe pump and this involves carryover,slow throughput, limited volume and wasted sample problems.

The use of a peristaltic pump according to the present invention in asystem for delivering a sample for analysis to spectroscopic apparatusoffers the advantage that a more even flow of sample into the apparatusis achievable than is the case with prior such systems.

Thus an object of a further aspect of the present invention is toprovide an improved method and apparatus for preparing and introducingsolutions for analysis.

Accordingly the invention also provides a system for delivering a samplefor analysis to spectroscopic apparatus, including means for supplying astream of sample solution, means for supplying a stream of diluent,means for combining the two streams, and means for delivering thecombined stream into a nebulizer of the spectroscopic apparatus at asubstantially constant flow rate, wherein the means for supplying astream of sample solution or the means for supplying a stream of diluentincludes a peristaltic pump according to the invention, and the systemis arranged such that on variation of the flow rate of the stream ofsample solution or the stream of diluent, the other stream (of diluentor sample solution) is automatically varied to maintain saidsubstantially constant flow rate into said nebulizer.

The invention also provides a method of spectroscopic analysis, whereina system as just described is used to supply a sample to a nebulizer ofspectroscopic apparatus for calibration of that apparatus and foranalysis of the sample, the method including supplying streams of samplesolution from a single on-line source of the sample solution.

In a method and apparatus according to the further aspect of theinvention, separate streams of sample solution and diluent respectivelyare combined and then introduced into the nebulizer as a single stream.The flow rate of the combined stream into the nebulizer is fixed by thenatural aspiration rate of the nebulizer, and the cross-sectional sizeof the passage through which the combined stream enters the nebulizer.The sample solution and the diluent are fed separately to a junction(that is a mixing point) at which the combined stream is formed, andeither the sample solution or the diluent is delivered to that junctionby a variable speed peristaltic pump according to the invention.Preferably, the pump delivers the sample solution to the junction, anddiluent is induced into the stream leaving the junction by a pressuredifferential existing between the nebulizer and the junction. As theflow via the peristaltic pump changes, the flow of the diluent streamautomatically changes to compensate. This changing flow results in achanged pressure drop between the diluent reservoir and mixing point.Such a pressure change could alter the pressure differential at whichthe nebuliser operates and thus alter its uptake rate, affecting overallsystem performance. To avoid this happening, it is desirable to ensurepressure changes at the mixing point are small relative to the pressuredrop between the mixing point and the nebuliser. This can be achieved byensuring the tubing bore between the mixing point and reservoir issubstantially larger than between the mixing point and nebuliser. Toachieve 1% accuracy, a ratio of bore diameter of at least 3:1 isdesirable, with higher ratio's preferable.

Assuming the maximum flow from the junction to the nebulizer is fixed atF1, and the flow rate of sample solution to the junction is F2, which isless than F1, diluent will be induced to flow into the junction at rateequivalent to F1 minus F2. Variation of F2 by appropriate adjustment ofthe pump then automatically results in a variation in the diluent flowrate, and consequently the dilution ratio of the stream entering thenebulizer.

In a preferred form of the system and method, standard solution is ableto be introduced into the aforementioned junction under the influence ofa second peristaltic pump according to the invention. Such anarrangement enables standard additions to be carried out in a simple andeffective manner as hereinafter described.

DESCRIPTION OF DRAWINGS

It will be convenient to hereinafter describe the invention in furtherdetail by reference to example embodiments which are shown in theattached drawings. The particularity of those drawings and theassociated description is not to be understood as superseding thegenerality of the preceding broad description of the invention accordingto each of its aspects.

FIG. 1 is a diagrammatic view of a peristaltic pump according to oneembodiment of the invention in which the presser plate is in the pumpinoperative position.

FIG. 2 is a view similar to FIG. 1 but showing the presser plate in thepump operative position.

FIG. 3 is an enlarged view of part of the assembly shown in FIG. 2.

FIG. 4 is a diagrammatic view of one arrangement for driving the presserplate of the pump shown in FIGS. 1 and 2.

FIG. 5 is a view similar to FIG. 4 but showing an alternative drivearrangement.

FIG. 6 is a side elevation view of yet another drive arrangement for thepresser plate.

FIG. 7 is an enlarged view of one form of presser plate suitable for usein the pump of FIGS. 1 and 2.

FIG. 8 is a view similar to FIG. 2, but in which the peristaltic pumpincludes camming surfaces in place of rollers.

FIG. 9 is a diagram of a system for delivering a sample to spectroscopicapparatus, and which represents an example use of a pump according tothe present invention.

FIG. 10 is a diagrammatic representation of a modified system to thatshown in FIG. 9, which allows standard additions to be carried out.

DETAILED DESCRIPTION OF EMBODIMENTS

The example pump 1 shown in FIG. 1 includes a rotatable drum 2 having abody 3 arranged for rotation about an axis 4, and a plurality of rollers5 attached to the drum body 3 so as to extend as a continuous seriesaround the periphery of that body 3. Each roller 5 is arranged to rotaterelative to the drum body 3 about its own individual axis 6. Anysuitable drive means may be provided to cause rotation of the drum 2.

A presser plate 7 is mounted on a support (not shown) so as to berotatable about an axis 8 to adopt either a pump inoperative position ora pump operative position, which are shown in FIGS. 1 and 2respectively. A flexible tube 9 is interposed between an operativesurface 10 of the presser plate 7 and the periphery of the drum 2. Whenthe presser plate 7 is in the operative position as shown in FIG. 2, aportion of the tube 9 which is trapped between the surface 10 and therollers 5 is distorted as shown in FIG. 3. A pinch zone 11 is therebycreated between each roller 5 and the plate surface 10 so that a body ofliquid 12 within the tube 9 is trapped between each two adjacent zones11. As the drum 2 rotates relative to the tube 9 the zones 11 move alongthe length of the tube 9 and thereby move the liquid bodies 12 in thesame direction.

The tube 9 may be held against movement with the drum 2 in any suitablefashion. It is a characteristic of the particular construction shownhowever, that a section 13 of the tube 9 is held at two supports 14 and15, and that the support 14 is attached to or formed integral with aportion 16 of the presser plate 7 which is located remote from the plateaxis 8. The arrangement is such that movement of the plate 7 about theaxis 8 causes the plate portion 16 to move towards or away from theperiphery of the drum 2, and the tube support 14 is moved accordingly.In the particular arrangement shown, the other tube support 15 is alsoattached to or formed integral with the presser plate 7, but at alocation adjacent the axis 8 so that there is relatively little movementtowards and away from the drum 2.

Any suitable means may be utilised to hold the tube 9 at each of thesupports 14 and 15. By way of example, two saddles 17 may be secured(for example by an adhesive) to the tube 9 against relative movement andarranged in spaced relationship such that each is cooperable with arespective one of the supports 14 and 15. Each support 14,15 is formedby a channel 60 in an end face of presser plate 7 and an intersectinggroove 61 (see FIG. 7).

Movement of the presser plate 7 about the axis 8 can be achieved throughuse of any suitable drive means. It is preferred however, that anelectro-mechanical actuator is used for that purpose. By way of example,the actuator may include a solenoid 18 as shown in FIG. 4 which isconnected between the presser plate 7 and a support 19. In theparticular arrangement shown in FIG. 4, the solenoid 18 operates whenenergised to move the plate 7 to the pump operative position of FIG. 2,and a spring 20 functions to move the plate 7 to the pump inoperativeposition when the solenoid 18 is de-energised. In an alternativearrangement which is not shown, a spring could move the plate 7 to theoperative position and a solenoid could operate to move the plate 7 tothe inoperative position.

FIG. 5 shows another possible drive arrangement for the plate 7 in whicha cam 21 is caused to rotate about an axis 22 by means of a suitabledrive motor (not shown). The arrangement is such that the rotationalposition of the cam 21 determines the position of the plate 7 relativeto the drum 2. Any suitable means, such as a spring (not shown), may beused to hold the plate 7 in contact with the cam 21.

A preferred drive arrangement for the plate 7 is shown diagrammaticallyin FIG. 6. According to that arrangement, the plate 7 is mounteddirectly on to the shaft 23 of a drive motor 24, which could be a gearmotor or a rotatable solenoid, for example. The bearings which supportthe shaft 23 determine the pivot axis 8 of the plate 7, and the plate 7is connected to the shaft 23 so that rotation of the shaft 23 causesmovement of the plate 7 between the pump operative and inoperativepositions. Such an arrangement is extremely simple and involves aminimum number of mechanical parts. The arrangement may be such that thedrive motor 24 stalls at the pump operative position, and consequentlyhas the characteristic of a current to torque converter when driven at acontrolled current. Thus, the torque and consequently the force appliedby the plate 7 to the tube 9, can be controlled by variation of thecurrent applied to the motor 24 by controller 25. Any suitable means maybe adopted to guard against overload of the motor 24.

It will be appreciated that when the FIG. 6 arrangement is used in theassembly of FIGS. 1 and 2, it permits automatic loading and unloading ofthe tube 9, thereby overcoming a major problem with prior peristalticpumps. In particular, the tube 9 is not left in a stressed condition assometimes happens with prior pumps, and therefore has an extended usefullife, although in a preferred construction of the present invention, thetube 9 is only minimally tensioned, as described above.

The operative surface 10 of the presser plate 10 may be profiled in amanner such as to either eliminate pulsations in the output flow, orminimise the adverse consequences of any such pulsations. That profilingmay be such that the surface 10 has two distinct regions, a pinch region25 and an expansion region 26, both of which are shown in FIG. 7. In thepreferred arrangement shown in FIG. 7 however, there is a third region27 which will be referred to as the entrance region. The expansionregion 26 is located downstream of the pinch region 25 relative to thedirection of flow through the tube 9, and the entrance region 27 islocated upstream of the pinch region 25.

In the particular arrangement shown in FIG. 7, the pinch region 25extends through an arc of approximately 24°, the extremities of whichare defined by lines 28 and 29 which extend radially from a point 30. Itis preferred that the surface region 25 is of uniform radius, the centerof which is located at the point 30. Furthermore, in the installedcondition of the presser plate 7, the point 30 is preferablysubstantially coincident with the axis 4 about which the drum 2 rotates.The arcuate length of the pinch region 25 may be determined by thenumber of rollers 5 (that is, 24° is appropriate for a pump having 15rollers 5) but may vary according to requirements, and consequently the24° extant of the arrangement shown should not be understood as criticalor essential.

A feature that may be incorporated in the pump is to arrange for thetube 9 to be "oversquashed" in a portion or all of the pinch region,that is, to arrange for the tube walls to be squeezed together slightlybeyond the limit necessary to effect a seal. This feature admits ofgreater manufacturing and assembly tolerances for the pump parts.

The expansion region 26 also follows a curved path, but is arranged sothat the distance between that path and the periphery of the drum 2progressively increases in the downstream direction. As previouslyindicated, the profile of the surface region 26 is preferably designedto achieve a substantially linear relationship between the angularrotation of the drum 2 and the increasing internal volume of the tube 9which follows withdrawal of the rollers 5 from contact with the tube 9.One possible approach is to construct a linear expansion profile of thesurface region 26 from the equation R=RO+KA, where:

R is the radius of curvature of the surface region 26 at a particularpoint in that region,

RO is the radius of curvature of the pinch region 25,

K is a constant defining the rate of expansion of the tube 9, and

A is the angle of rotation of the drum 2 beyond the point at which thesurface region 26 commences.

The extent to which pulsations are reduced depends upon the valueselected for K. A flow pulsation of approximately 10% is achievableusing an optimal value for K for the roller system that is employed.

If a parabolic expansion curvature is selected for the region 26, flowpulsations in the region of 8.5% may be achieved, whereas an exponentialprofile can achieve a better result with residual pulsations in theorder of 5.7%. A satisfactory profile, and the equation for generatingthat profile, can be determined according to individual needs.

In the particular arrangement shown in FIG. 7, the extremities of theexpansion region 26 are defined by the lines 28 and 31 which extendradially from the point 30. The angular extent of the surface region 26is approximately 66° in the arrangement shown, but a different angularextent may be selected.

The surface region 27 is preferably arranged to achieve progressivecompression of the tube 9 as the drum 2 advances over the tube section13. Any suitable curvature can be selected for that purpose, subjectonly to the requirement that the separation between the surface region27 and the drum periphery decreases in the downstream direction. Theangular extent of the region 27 is defined by the radial lines 29 and32, and in the example shown it is approximately 66°, that is, equal tothe expansion region.

It is preferred to provide a membrane 33 (FIG. 1) between the drum 2 andthat part of the tube 9 which is subjected to the influence of thepresser plate 7. The upstream end 34 of the membrane 33 may be anchoredin any appropriate fashion so as to be fixed against movement with thedrum 2, and the anchoring point 35 need not be located as shown inFIG. 1. The downstream end 36 of the membrane 33 may be anchored also,but preferably in such a way as to allow some movement of that end inthe longitudinal direction of the tube 9.

The purpose of the membrane 33 is to absorb shear forces generated byfrictional resistance to rotation of the rollers 5 and thereby protectthe tube 9 against longitudinal stretching. It is desirable that themembrane 33 be sufficiently flexible not to hinder expansion of the tube9 into the region between adjacent rollers 5 as shown in FIG. 3. It isalso desirable that the membrane 33 be resistant to stretching in thelongitudinal direction of the tube 9 when subjected to the forcesgenerated by the presser plate 7 in the pump operative position.Suitable materials for the membrane 33 include plastics films such aspolyester. Mylar having a thickness in the range of 0.1 millimeter to0.2 millimeter has been found to be satisfactory.

When the presser plate 7 is in the operative position, the membrane 33is sandwiched between the rollers 5 and the operative surface 10 of theplate 7. Thus, the membrane 33 supplies the forces required to overcomefrictional resistance to rotation of the rollers 5, and thereby absorbsthe associated shear forces. Since the membrane 33 does not stretch, allshear and tension forces are eliminated from the tube 9, which is heldin its operative position between supports 14 and 15 such that virtuallya zero tension force is applied to it between those supports. That notonly eliminates flow pulsations induced by longitudinal stretching ofthe tube 9, but also leads to longer tube life and more stable flowcharacteristics because of the elimination of shear force fatiguefailures.

The example pump 1' shown in FIG. 8 is similar to the pump described inFIGS. 1-7 (note that the same reference numerals, but with a prime, havebeen used in FIG. 8 to denote features that correspond in the twoembodiments) except that instead of having a rotatable drum and rollers,it includes a drum in the form of a cam 3', arranged for rotation aboutan axis 4'. Cam 3' has a plurality of camming surfaces 5' extending as acontinuous series around its periphery. The number of camming surfaces5' on cam 3' may be chosen to optimise flow linearity, for example, thenumber of surfaces 5' depicted in the FIG. 8 embodiment, namely 15, maybe increased to reduce the magnitude of each flow pulsation, althoughthe frequency of the pulses will be increased. Each surface 5' iscircular in profile, although it is within the scope of the invention toemploy other than a circular profile for the camming surfaces.

A membrane 33' is provided between the cam 3' and that part of the tube9' which is subjected to the influence of the presser plate 7'. As inthe FIGS. 1-7 embodiment, the purpose of the membrane 33' is to absorbshear forces generated by frictional resistance to rotation of the cam3' and thereby protect the tube 9' against longitudinal stretching.

Suitable materials for the membrane 33' include plastics films such aspolyester, particularly materials which are highly slippery so as toreduce frictional forces between the camming surfaces 5' and themembrane. An example of a particularly suitable material is Ultra HighMolecular Weight Poly Ethylene (UHMWPE). Membrane 33' may be a laminateof Mylar and UHMWPE, and be positioned such that the UHMWPE faces thecamming surfaces 5' in order to minimize the frictional force betweenthe membrane and surfaces 5' as they slide along the membrane.Preferably the membrane is such as to consist entirely of the onematerial.

It should be noted that as the tube 9 ages, if some extension of it doesoccur due to the cumulative effects of squashing pressure being appliedby the presser plate and compressing elements, the tube supports 14 and15 as shown in the figures are such as will allow for any such increasein tube length while still correctly holding the tube in its operatingposition.

According to a preferred feature of the invention a cyclically varyingholding force is applied to the presser plate 7 or 7' by anelectro-mechanical actuator. This may be done by varying the electricalpower supplied to the actuator from a control system (not shown),cyclically at a convenient frequency such that the maximum currentvariation from a mean is that current which approaches that required toovercome the internal friction of the actuator. This is known as"dithering".

A peristaltic pump as described is ideally suited for use inspectroscopic apparatus for delivering a sample to the nebulizer of suchapparatus. An example arrangement of that kind is shown diagrammaticallyby FIG. 9.

In the FIG. 9 arrangement, a nebulizer 49 is shown which forms part ofspectroscopic apparatus 38. Nebulizer 49, a sample solution supply 50and a diluent supply 51, are each separately connected to a junction 52.The connection between the junction 52 and the nebulizer 49 is by way ofa feed passage 53 which is preferably of relatively smallcross-sectional size for a reason hereinafter made clear. The samplesolution supply 50 is connected to the junction 52 through a deliverypassage 54 and a peristaltic pump 55 (as previously described inrelation to FIGS. 1-7 or FIG. 8) having its output side connected tothat passage. The diluent supply 51 is connected to the junction 52through a passage 56 which preferably has a relatively largecross-sectional size by comparison with that of the feed passage 53.

It is a feature of the arrangement shown that the pump 55 iscontrollable to deliver sample solution through the passage 54 at any ofa variety of precisely controllable flow rates.

Because of the relatively small size of the feed passage 53, it exhibitsa high resistance to flow. The passage 56 on the other hand, exhibitsrelatively low resistance to flow. As a consequence of that difference,substantially all the pressure drop in the system illustrated occursbetween the nebulizer 49 and the junction 52. A nebulizer of the kindused in spectrophotometers will usually generate a stable pressurereduction at the liquid intake port, and in that event the flow ratethrough the passage 53 will be substantially constant and stable.

If the pump 55 is operated to deliver sample solution to the junction 52at a flow rate slightly below the natural aspiration rate through thefeed passage 56, the solution will enter the nebulizer 49 in almostundiluted form. Assuming that the solution is found to have aconcentration above the desirable range, the speed of the pump 55 can bereduced to thereby reduce the flow rate through the delivery passage 54and consequently cause increased diluent to be induced into the streamflowing through the feed passage 56. Thus, the pump speed can beadjusted to obtain a satisfactory dilution ratio in the stream enteringthe nebulizer 49.

The sample concentration in the atomized body which is measured by thespectrometer, is thereby directly proportional to the rate of flowthrough the delivery passage 54, which is in turn proportional to thespeed of operation of the pump 55. It is therefore possible toaccurately and quickly adjust the dilution ratio as required.

If operation of the pump 55 is stopped for any reason, such as to changethe sample solution, the nebulizer 49 will continue to aspirate diluentfrom the supply 50. As a result, the nebulizer and associated spraychamber do not dry out as occurs in conventional systems, but aresubjected to rinsing by the clean diluent. That maintains the nebulizerand spray chamber in ideal condition for the next sample measuringoperation.

Since the diluent will usually have zero absorbance, a system asdescribed above can carry out an auto zero function during the time thatdiluent alone is passing through the nebulizer. That has the advantageof improving the system performance and simplifying user interaction.

The system described can also be used for micro sampling. The pump 55can be briefly operated to inject a small quantity of sample solutioninto the diluent stream passing through the feed passage 53. There is nochange in the flow through the nebulizer during that micro samplingoperation, and consequently the nebulizer characteristics remain stable.That is in contrast with conventional micro sampling involving briefintroduction of the sampling tube into the sampling liquid while thenebulizer aspirates air immediately before and immediately after thatintroduction of the sampling tube.

The use of a peristaltic pump, because of its flow throughcharacteristics, means there are no separate fill or flush stages aswould be necessary with a syringe pump and which slow down the operatingsequence. Furthermore, a multi channel peristaltic pump could be used soas to simultaneously pump several liquids each of which has a fixed flowrate determined by the cross-sectional size of the respective channel(tube) through which it flows. By way of example, use of three tubes orchannels for sample, acidifier and sodium borohydride respectively,makes it possible to use the system described for hydride operations.

FIG. 10 shows a modified form of the system described in relation toFIG. 9 in which there is added a second pump 57 connected to a supply 58of standard solution and also connected to the junction 52 through adelivery passage 59. The pump 57 is a variable speed peristaltic pump asdescribed in relation to FIGS. 1-7 or FIG. 8 so that control of therelative speeds of the two pumps 55 and 57 enables an accurate ratio ofsample to standard to be maintained.

The modified system is intended for use in relation to the well knownstandard additions technique, which involves measuring the sample andthen measuring the sample spiked with an additional known amount ofanalyte. It is possible to determine the concentration of analyte in theoriginal (non-spiked) sample by comparing the two measurements.

Standard additions as carried out in conventional systems has a numberof deficiencies. By way of example, a linear regression line is usuallydrawn through the spiked and unspiked absorbance readings. That assumesthe existence of a linear calibration curve, and yet it is known that AAcalibration curves are not linear at high concentrations Of analyte.Although several samples spiked to different degrees could be analysedand used to compute a non linear calibration curve, the amount ofoperator time and effort necessary to prepare and measure the sampleswould be significant and is therefore seldom done.

Furthermore, spiking the sample changes the ratio of matrix to analyteconcentration and therefore changes the effect of the matrix. In orderto minimise that result, it is desirable that the spike only increasethe total analyte concentration by a reasonable amount. At the sametime, a small spike may yield poor precision in the measurement resultsbecause of the small difference between the spiked and unspikedreadings. In conventional standard additions analysis however, both theunspiked and spiked samples need to be available at the time of theanalysis and therefore the spiked sample must be prepared withoutknowing even the approximate concentration of the analyte in the sample.This means that the change in concentration introduced by the spikingcould be very large (e.g., 10 or even 100 times for low concentrationsamples), or conversely very small for high concentration samples. Inorder to overcome that problem, it is necessary to first analyse thesample by conventional calibration to obtain an approximate analyteconcentration, and use that information to determine the amount ofspiking. Such an approach necessarily doubles the operator work load.

In addition to the foregoing, standard additions using conventional AAinstrumentation is extremely slow, operator intensive and prone toerror. Whereas in the case of normal calibration the operator onlyprepares the sample and can analyse the sample in a few seconds, forstandard additions the operator must prepare and analyse at least oneadditional solution (and often more than one) involving accurateweighing, measuring and mixing. The time per sample is increased from afew seconds to several minutes.

A system of the general kind diagrammatically illustrated in FIG. 10 hasthe benefit of overcoming or at least minimising the aforementionedproblems. That is particularly the case by use of a method as describedbelow.

A preferred method using the system of FIG. 10 involves the followingsteps:

(a) measure the sample,

(b) if the measured absorbance is above 50% of the linear range, dilutethe sample,

(c) measure the diluted sample,

(d), add a standard to the sample so as to approximately double theabsorbance,

(e) measure the spiked sample, and

(f) compare measurement (e) with measurement (a) or (c) as appropriate.

In the event that dilution of the sample is not required, themeasurement comparison will of course be between the measurementreferred to under (a) above and the measurement referred to under (e).In the case of a diluted sample the comparison will be between themeasurement referred to in (c) above and the measurement referred to in(e).

The method described above has the benefit that the operator is onlyrequired to present a single sample as normal calibration, and becauseall dilutions and additions are carried out in line it is possible toachieve a high throughput.

It will be apparent from the foregoing description that the systemaccording to the present invention provides improvements in both normalcalibration and standard additions. Operator time is minimised withoutsacrifice of accuracy in the results of the analysis.

It will be also evident from the foregoing that a pump according to thepresent invention overcomes substantial problems existing in priorperistaltic pumps, and in particular provides a peristaltic pump whichis suitable for use in situations requiring a stable and accurate flowrate. Furthermore, a peristaltic pump as shown in FIG. 8 is of moresimple construction, having fewer moving parts (thus providing amechanism that is less prone to corrosion) and in which the need forvery small manufacturing and assembling tolerances is reduced.

Finally, it is to be understood that various alterations, modificationsand/or additions may be introduced into the constructions andarrangements of parts previously described without departing from thespirit or ambit of the invention as defined in the appended claims.

We claim:
 1. A peristaltic pump comprising a rotatable drum includingcompressing elements around its periphery, a flexible tube extendingbetween two spaced supports and a movable presser plate for holding thetube against the periphery of the drum between the two spaced supportssuch that on rotation of the drum the compressing elements squeeze thetube against the presser plate for moving fluid through the tube, saidpresser plate having a pinch region having a surface of curvature R_(o)wherein said tube is squeezed to a condition of closure and an expansionregion of said plate having a variable curvature R such that R=(KR_(o))Awhere A is the angular rotation of said drum within said expansionregion thereby producing a linear expansion of said flexible tube andthe arcuate length of said expansion region being greater than that ofsaid pinch region, wherein at least one of the tube supports is movablerelative to the drum to establish an operative and an inoperativeposition for the tube, the operative position being when the tube ispositioned for pumping.
 2. A peristaltic pump as claimed in claim 1,wherein the said one movable support is fixed to the presser plate formovement therewith to establish said operative and inoperative positionsfor the tube.
 3. A peristaltic pump as claimed in claim 1, wherein thecompressing elements are camming surfaces on the periphery of the drum.4. A peristaltic pump as claimed in claim 1, wherein the compressingelements are rollers, rotatably mounted around the periphery of thedrum.
 5. A peristaltic pump as claimed in claim 4 or claim 3, wherein aflexible membrane is interposed between the tube and the drum, theconstruction and/or material of the membrane being such as to eliminateall shear and tension forces from the tube due to the action of thecompressing elements thereon on rotation of the drum.
 6. A peristalticpump as claimed in claim 1, wherein the profile of the said surface alsoincludes an entrance region, for progressive compression of the tube,and a pinch region, at which the tube is squeezed closed, preceding theexpansion region.
 7. A perstaltic pump as claimed in claim 6, whereinthe entrance and expansion regions are of substantially equal angularextent and the angular extent of the pinch region equals approximately360° divided by the number of compressing elements.
 8. A peristalticpump as claimed in claim 1, wherein the presser plate is pivotallymounted and both of said supports are fixed thereto, one of saidsupports being fixed in proximity to the pivot axis of the presser platesuch that that support is substantially immovable relative to the drum.9. A peristaltic pump as claimed in claim 8, wherein anelectromechanical actuator is operably connected to the presser platefor moving the presser plate between the inoperative and operativepositions and for holding the presser plate in its operative position.10. A peristaltic pump as claimed in claim 9, further including acontrol system operably associated with the presser plate forautomatically moving the presser plate to move the tube between itsoperative and inoperative positions.
 11. A peristaltic pump as claimedin claim 9, wherein the electromechanical actuator is operable to applya cyclically varying force to the presser plate while holding thepresser plate in the operative position, whereby the average of thevarying force is such as to maintain the presser plate in positionagainst the average of forces exerted thereon by rotation of the drum.